1. Field of the Invention
The present invention relates to a magneto-optical recording medium. In particular, it relates to a magneto-optical recording medium of the type which shows improved sensitivity to the data-recording magnetic field and also exhibits improved readout C/N characteristics.
2. Description of the Related Art
Conventionally, various kinds of magnetic media that can be written to repeatedly (such as hard disks, floppy disks, magneto-optical disks and tapes) have been widely used for storing data. Among them, magneto-optical disks (simply called xe2x80x9cMO disksxe2x80x9d hereinafter) are advantageous in data retention since they will deteriorate (demagnetize) more slowly than the other kinds of magnetic media mentioned above.
As is known, a technique called xe2x80x9cMagnetically-Induced Super Resolution (MSR)xe2x80x9d may be used for realizing a high-density data-reproducing (readout) performance. According to this technique, data can be read out from an MO disk in which minute recording marks are arranged along the tracks by a pitch which is smaller than the diameter of the spot of the laser beam irradiated on the MO disk.
An MSR technique is disclosed in JP-A-7(1995)-244877 for example. This document teaches the use of xe2x80x9cdouble-mask RAD (Rear Aperture Detection)xe2x80x9d for reading data from an MO disk. By this method, cross talk between adjacent tracks can be minimized.
Referring to FIGS. 9-11 of the accompanying drawings, the double-mask RAD method and the typical layer structure of an MO disk used for implementing this method will be described below.
As shown in FIG. 9, the conventional MO disk includes a readout layer 31, an intermediate layer 32 and a data-recording layer 33. These layers, which are made of a rare earth-transition metal amorphous alloy, are supported by a transparent resin substrate 20 made of e.g. polycarbonate. Thought not illustrated, the MO disk includes additional layers made of SiN (silicon nitride) for protection purposes. Laser beams, emitted on the side of the transparent substrate 20 for erasing, recording or reading data, will pass through the substrate 20 and strike on the readout layer 31.
The readout layer 31 has a transition metal magnetization-dominant composition (hereinafter called xe2x80x9cTM-richxe2x80x9d composition). The direction of magnetism of the layer 31 is oriented perpendicularly to this layer. The intermediate layer 32 has a rare earth magnetization-dominant composition (hereinafter called xe2x80x9cRE-richxe2x80x9d composition). At room temperature, the direction of magnetism of the intermediate layer 32 is oriented longitudinally thereof, whereas at higher temperatures, the direction is perpendicular. This means that the direction of magnetism of the intermediate layer 32, which is longitudinal at room temperature, is changed to be perpendicular at a certain temperature higher than the room temperature. The data-recording layer 33 has a TM-rich composition, and its direction of magnetism is perpendicular.
When the Curie temperatures of the three layers 31, 32 and 33 are Tc1, Tc2 and Tc3, respectively, the following relations hold:
Tc2 less than Tc1 and Tc2 less than Tc3.
Further, when the coercivities of the readout layer 31 and the data-recording layer 33 are Hc1 and Hc3, respectively, the following relations hold:
Hc3 greater than Hc1.
The deletion of recording marks formed on the magneto-optical disk 10 is performed in the following manner. First, as shown in FIG. 10, the readout layer 31 is irradiated with a laser beam when the MO disk is held in a downward, data-deleting magnetic field. Thus, the irradiated area on the layer 31 and portions of the three layers 31-33 adjacent to the irradiated area are heated up to a temperature above the the Curie temperature Tc3. In this state, the direction of the magnetic domains on the data-recording layer 31 is aligned with the direction of the data-deleting magnetic field.
Then, the heated portions of the three layers 31-33 are brought away from the laser beam to cool down to the room temperature. Consequently, the direction of the cooled magnetic domains on the intermediate layer 32 becomes longitudinal of the layer 32 (horizontal in the figure). Thereafter, the readout layer 31 and the data-recording layer 33 are magnetically bonded to each other by weak force. Thus, all of the magnetic domains of the respective layers 31 and 33 are aligned in one direction (downward in FIG. 10). This means that the previous data stored in the data-recording layer 33 has been deleted.
Once the previous data is deleted, new data can be written to the MO disk by a Light Intensity Modulation (LIM) method. Specifically, as shown in FIG. 11, the readout layer 31 is irradiated with a laser beam when the MO disk is put in an upward, data-recording magnetic field. According to the LIM method, the intensity of the laser beam is modulated in accordance with the data to be recorded, while the MO disk is kept in the data-recording magnetic field. Thus, only when the intensity of the laser beam is high, the direction of the irradiated magnetic domains on the recording magnetic layer 33 is aligned with the upward data-recording magnetic field. In this manner, appropriate recording marks are produced.
When the laser-heated region on the MO disk is brought away from the laser beam, it cools down to the room temperature. As a result, the direction of the cooled magnetic domain of the intermediate layer 32 is horizontally directed, thereby rendering the readout layer 31 and the data-recording layer 33 magnetically bonded to each other with weak force.
In the above state, the direction of the magnetic domains of the readout layer 31 can be aligned in one direction upon application of an external magnetic field which is stronger than the above-mentioned weak magnetic force bonding the readout layer 31 to the data-recording layer 33.
Instead of the LIM method described above, Magnetic Field Modulation (MFM) may be used for writing data to the MO disk. According to this method, the direction of the applied magnetic field is modulated (reoriented upward and downward repeatedly) in accordance with the data to be recorded, while the readout layer 31 is being irradiated with a laser beam. In this manner, the direction of the laser-irradiated magnetic domains is aligned with the direction of the applied magnetic field.
The thus recorded data is read out from the MO disk in the following manner. As shown in FIG. 9, the readout layer 31 is irradiated with a laser beam S, while the laser-irradiated region is put in a downward, data-readout magnetic field. In the low-temperature region (hatched region) corresponding to a front portion of the laser beam S, the intermediate layer 32 and the data-recording layer 33 are only weakly bonded to each other, so that the direction of the magnetic domains of the intermediate layer 32 is aligned with the externally applied readout magnetic field. Then, due to the exchange interaction between the intermediate layer. 32 and the readout layer 31, the direction of the magnetic domains of the readout layer 31 is directed upward. In this manner, the direction of the magnetic domains of the data-recording layer 33 is masked (front mask) by the readout layer 31.
The temperature of the high-temperature region (crosshatched region) corresponding to a rear portion of the laser beam S is higher than the Curie temperature of the intermediate layer 32, thereby breaking the exchange interaction between the intermediate layer 32 and the readout layer 31. Thus, the direction of the magnetic domains of the readout layer 31 is aligned with the direction of the externally applied readout magnetic field. Thus, the direction of the magnetic domains of the data-recording layer 33 is masked (rear mask).
In the intermediate-temperature region between the low-temperature region and the high-temperature region (i.e., the region between the hatched region and the crosshatched region), the exchange interaction between the data-recording layer 33 and the readout layer 31 causes the direction of the magnetic domains of the readout layer 31 to be copied to the magnetic domains of the data-recording layer 33.
As described above, when the MO disk is irradiated by the laser beam for reading out the stored data, both the low-temperature region and the high-temperature region within the laser spot S are masked, while the intermediate-temperature region is left unmasked. This intermediate-temperature region serves as an aperture from which the recorded data can be read out. As viewed along the tracks on the MO disk, the aperture has a length L which is smaller than the diameter of the laser spot S. Thus, it is possible to read the recorded data based on the minute recording marks arranged in the track by a pitch smaller than the diameter of the laser spot S. Further, the width W of the intermediate-temperature region is smaller than the width (diameter) of the laser spot S, whereby the cross talk between adjacent tracks is advantageously reduced.
To improve the performance of the MO disk described above, it is necessary to improve the sensitivity to the recording magnetic field and also to improve the readout C/N characteristics. When the sensitivity to the recording magnetic field is improved, it is possible to use a weaker magnetic field for recording and deleting data by the LIM method. The use of a weaker magnetic field leads to the downsizing of the unit and improvement of the energy efficiency. When the data-recording is performed by the MFM, the improvement of the sensitivity to the recording magnetic field will result in proper recording mark formation even if the recording magnetic field is weakened by drop in effective current due to high frequency loss. Thus, with the improvement of the sensitivity to the recording magnetic field, it is possible to perform high density data-recording by increasing the frequency of the recording magnetic field.
JP-A-5(1993)-298764 discloses an MSR MO disk by FAD (Front Aperture Detection). This conventional MO disk is provided with a recording layer made of magnetic materials which have an RE-rich composition at room temperature. A magnetic layer is formed on the recording layer for improving the sensitivity to the recording magnetic field. This magnetic layer is made of materials which have a TM-rich composition at a temperature of forming recording marks on the recording layer.
However, according to the teaching of JP-A-5(1993)-298764, it is impossible to satisfactorily reduce the length of the data-reading aperture along the tracks since the conventional MO disk employs the FAD method. Therefore, even if the sensitivity to the recording magnetic field is improved and high-density formation of recording marks by magnetic field modulation is possible, the high-density recording marks cannot be properly read out from the disk. Further, in the conventional MO disk, the readout C/N is not improved.
It is, therefore, an object of the present invention to provide a magneto-optical recording medium capable of forming recording marks with higher density by magnetic field modulation.
Another object of the present invention is to make it possible to properly read out the high-density recording marks from the MO recording medium by MSR.
Still another object of the present invention is to improve the readout C/N with the MO recording medium.
According to a first aspect of the present invention, there is provided a magneto-optical recording medium for super resolution readout by double mask RAD. The MO recording medium includes four magnetic layers, namely, first to fourth magnetic layers. The first magnetic layer serves as a readout layer and is made of a rare earth-transition metal amorphous magnetic material. The Curie temperature of the first magnetic layer is Tc1.
The second magnetic layer serves as an intermediate layer and is made of a rare earth-transition metal amorphous magnetic material. The second magnetic layer has a rare earth metal magnetization-dominant composition and an axis of easy magnetization which is oriented longitudinally of the second magnetic layer at room temperature. The second magnetic layer has no compensation temperature below a Curie temperature Tc2 of the second magnetic layer.
The third magnetic layer serves as a recording layer and is made of a rare earth-transition metal amorphous magnetic material. The third magnetic layer has a transition metal magnetization-dominant composition or compensation composition at room temperature. The Curie temperature of the third magnetic layer is Tc3.
The fourth magnetic layer is formed on the third magnetic layer and is made of a rare earth-transition metal amorphous magnetic material containing at least Gd. The fourth magnetic layer has, at room temperature, a rare earth metal magnetization-dominant composition and an axis of easy magnetization which is oriented longitudinally of the fourth magnetic layer. The axis of easy magnetization of the fourth magnetic layer is altered in direction to be oriented perpendicularly to the fourth magnetic layer as the temperature of the fourth magnetic layer rises to the Curie temperature Tc4 of the fourth magnetic layer.
According to a preferred embodiment, the Curie temperatures Tc1, Tc2, Tc3 and Tc4 may be determined so that Tc2xe2x89xa6Tc4xe2x89xa6Tc3 less than Tc1, and Tc2 less than Tc3.
According to another preferred embodiment, the Curie temperatures Tc1, Tc2, Tc3 and Tc4 may be determined so that Tc2 less than Tc3xe2x89xa6Tc4xe2x89xa6Tc1, and Tc3 less than Tc1.
Preferably, the fourth magnetic layer may contain 25-35 at % of Gd. Further, the fourth magnetic layer may have a thickness of no greater than 20 nm.
According to a second aspect of the present invention, there is provided a magneto-optical recording medium for super resolution readout by double mask RAD. This MO recording medium includes four magnetic layers, namely, first to fourth magnetic layers. The first magnetic layer serves as a readout layer and is made of a rare earth-transition metal amorphous magnetic material. The Curie temperature of the first magnetic layer is Tc1.
The second magnetic layer serves as an intermediate layer and is made of a rare earth-transition metal amorphous magnetic material. The second magnetic layer has a rare earth metal magnetization-dominant composition and an axis of easy magnetization which is oriented longitudinally of the second magnetic layer at room temperature. The second magnetic layer has no compensation temperature below the Curie temperature Tc2 of the second magnetic layer.
The third magnetic layer serves as a recording layer and is made of a rare earth-transition metal amorphous magnetic material. The third magnetic layer has a transition metal magnetization-dominant composition or compensation composition at room temperature. The Curie temperature of the third magnetic layer is Tc3.
The fourth magnetic layer is formed on the third magnetic layer and made of a rare earth-transition metal amorphous magnetic material containing at least Gd. The fourth magnetic layer has, at room temperature, a transition metal magnetization-dominant composition and an axis of easy magnetization which is oriented perpendicularly to the fourth magnetic layer. The axis of easy magnetization of the fourth magnetic layer is altered in direction to be oriented longitudinally of the fourth magnetic layer as the temperature of the fourth magnetic layer rises to the Curie temperature Tc4 of the fourth magnetic layer.
Preferably, at about the Curie temperature Tc3, the third and the fourth magnetic layers may have coercivity Hc3 and coercivity Hc4, respectively. The coercivities Hc3, Hc4 and the recording magnetic field Hw may be determined so that Hc3 less than Hw and Hc4 less than Hw.
In the second aspect of the present invention again, the Curie temperatures Tc1, Tc2, Tc3 and Tc4 may be determined so that Tc2xe2x89xa6Tc4xe2x89xa6Tc3 less than Tc1, and Tc2 less than Tc3. Or, the Curie temperatures Tc1, Tc2, Tc3 and Tc4 may be determined so that Tc2 less than Tc3xe2x89xa6Tc4xe2x89xa6Tc1, and Tc3 less than Tc1.
Preferably, the fourth magnetic layer may contain 25-35 at % of Gd. Further, the fourth magnetic layer may have a thickness of no greater than 20 nm.
In an MO recording medium according to the present invention, the magnetic properties of each of the first to the third magnetic layers are determined so that super-resolution readout can be performed by the double mask RAD method.
Further, according to the present invention, a fourth magnetic layer is formed on the third magnetic layer (recording layer). Specifically, the third magnetic layer has two principal surfaces: a first surface closer to the transparent substrate carrying the stack of the first to the third magnetic layers; and a second surface farther from the transparent substrate. The fourth magnetic layer is arranged on the second surface of the third magnetic layer. With the fourth magnetic layer provided, the sensitivity to the recording magnetic field as well as the readout C/N is advantageously improved.
The reason why the sensitivity to the recording magnetic field is improved in the MO recording medium according to the first aspect of the present invention may be as follows. In magnetizing the third magnetic layer (recording layer) in the direction of the recording magnetic field, the axis of easy magnetization of the fourth magnetic layer is oriented perpendicularly to the fourth layer when the temperature of the fourth layer is raised to a certain point. Consequently, the third magnetic layer is put in the biased static magnetic field generated by the fourth magnetic layer. Thus, upon application of a relatively weak recording magnetic field, the magnetization of the third magnetic layer will reach its saturation point. This means that the sensitivity to the recording magnetic field is improved.
The reason why the readout C/N is improved in the MO recording medium according to the first aspect of the present invention may be as follows. In reading out the stored data from the MO recording medium, the first magnetic layer (readout layer) of the MO recording medium is irradiated with a laser beam. Thus, as previously stated, an intermediate-temperature region will be produced in the magnetic layers of the MO recording medium. Corresponding to this intermediate-temperature region, each of the third and the fourth magnetic layers is provided with an intermediate-temperature portion. It is possible, by having properly adjusted the Curie temperature Tc4 of the fourth magnetic layer, to align the magnetization direction of the intermediate-temperature portion of the third magnetic layer with the magnetization direction of the intermediate-temperature portion of the fourth magnetic layer. Consequently, the magnetic exchange bonding force between the first magnetic layer and the third magnetic layer via the second magnetic layer is enhanced. Thus, the magnetization direction of the third magnetic layer can be copied to the first magnetic layer more properly than is conventionally possible. This means that the readout C/N is improved.
The reason why the sensitivity to the recording magnetic field is improved in the MO recording medium according to the second aspect of the present invention may be as follows. In magnetizing the third magnetic layer (recording layer) in the direction of the recording magnetic field, the axis of easy magnetization of the fourth magnetic layer has a component which is longitudinal of the fourth layer when the temperature of the fourth layer is raised to a certain point. Consequently, the magnetic domains of the fourth layer are stabilized. Thus, upon application of a relatively weak recording magnetic field, the magnetization of the third magnetic layer will reach its saturation point. This means that the sensitivity to the recording magnetic field is improved.
The reason why the readout C/N is improved in the MO recording medium according to the second aspect of the present invention may be as follows. In reading out data from the MO recording medium, a properly-adjusted Curie temperature Tc4 of the fourth magnetic layer can cause the axis of easy magnetization of the intermediate-temperature portion of the fourth layer to be oriented longitudinal of the fourth layer. As a result, the magnetic domains of the fourth layer are rendered stronger, which in turn serves to enhance the magnetic exchange bonding force between the first magnetic layer and the third magnetic layer via the second magnetic layer. Thus, the magnetization direction of the third magnetic layer can be copied to the first magnetic layer more properly than is conventionally possible. This means that the readout C/N is improved.
In summary, according to the present invention, the sensitivity to the recording magnetic field is advantageously improved. Thus, high-density data recording by high-frequency magnetic field modulation can be performed, while also achieving improvement of the readout C/N. Further, when data-recording and data-deleting are performed by light intensity modulation, a weaker recording magnetic field suffices. This serves to downsize a magneto-optical disk apparatus, while also helping to save power supplied to the apparatus.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.